The present invention generally relates to semiconductor devices, and more particularly to a laser diode that produces an optical beam in a direction substantially perpendicular to the epitaxial layers that form the laser diode.
In the laser diode, the cleaved surface of the epitaxial layers is used generally for forming the optical cavity. In such ordinary laser diodes, therefore, the optical beam is produced in the direction substantially perpendicular to the cleaved surfaces and hence in the direction substantially parallel to the epitaxial layers. In the mass production of such ordinary laser diodes, there arises a problem in that each laser diode has to be tested individually. However, such a testing process takes a substantial time and increases the cost of the device.
On the other hand, there is proposed a laser diode called surface radiation type that produces the output optical beam in the direction substantially perpendicular to the epitaxial layers that form the laser diode. In this type of laser diode, it is possible to carry out the test of the devices in the state that the devices are formed on a wafer. More specifically, the test of the laser diodes is achieved efficiently by moving a probe of the testing machine over the surface of the wafer. Thereby, the cost of the device can be reduced substantially.
Such a surface radiation type laser diode is useful also in constructing various processing systems and computers by stacking a number of circuit boards that carry thereon various semiconductor devices. By using the surface radiation type laser diodes, the interconnection between the circuit boards can be achieved easily and efficiently by the optical beam. Further, by arranging such laser diodes in the form of a row and column array, one can represent an image by a luminescent pattern. Such an arrangement is useful in restoring an image that has experienced modification during transmission.
FIG. 1(A) shows a conventional surface radiation type laser diode 10 in the cross sectional view while FIG. 1(B) shows the same device in the plan view.
Referring to FIGS. 1(A) and 1(B), the laser diode includes a substrate 11 of an InP doped to the n.sup.+ -type. On the substrate 11, there is provided an active layer of undoped InGaAs grown epitaxially on the substrate 11. As shown in the plan view of FIG. 1(B), the active layer 12 has a cylindrical form with a reduced diameter for concentrating carriers therein to achieve an efficient stimulated emission. On the upper major surface of the active layer 12, there is provided a contact layer 13 of InP doped to the P.sup.+ -type. Further, the active layer 12 is supported laterally by an insulating layer 14 that may be a resin such as polyimide or a semiconductor material processed to become unconductive. Further, there is provided a ring-shaped ohmic electrode 15 on the upper major surface of the contact layer 13. There, the electrode 15 defines an exposed upper major surface of the layer 13. Similarly, there is provided another ohmic electrode 16 at the lower major surface of the substrate 11. As shown in the plan view of FIG. 1B), the electrode 16 includes two rectangular conductor pieces separated from each other in correspondence to the active layer 12. Thereby, there is defined an exposed lower major surface of the substrate 11 between the two electrode pieces 16a and 16b. The exposed upper major surface of the contact layer 13 as well as the exposed lower major surface of the substrate 11 provides a passage of the optical beam that is produced by the laser diode. Thereby, the optical beam exits in the direction perpendicular to the epitaxial layers.
In the laser diode of FIG. 1(A), it will be noted that there is formed a pair of opposing mirror surfaces respectively in correspondence to the interface between the substrate 11 and the active layer 12 and in correspondence to the exposed upper major surface of the contact layer 13. Thereby, there is formed an optical cavity in correspondence to the active layer 12 and the contact layer 13, and the optical radiation produced in the active layer 12 is amplified by the stimulated emission as it is reflected back and forth between the two opposing mirror surfaces. The optical beam thus produced as a result of the stimulated emission exits through the exposed upper major surfaces of the contact layer 13 as well as through the exposed lower major surface of the substrate 11 in the direction perpendicular to the epitaxial layers as already mentioned.
In the laser diode 10 shown in FIGS. 1(A) and 1(B), the difference in the refractive index between the substrate 11 and the active layer 12 is not large enough to cause a sufficient optical feedback. In order to obtain a more strong reflection, it is preferable to use the exposed lower major surface of the substrate 11 for the mirror. On the other hand, the distance between the two opposing mirrors of the optical cavity is determined by the wavelength of the optical radiation and thus, there can be a case in which the use of the lower surface of the substrate 11 as the mirror is not possible.
FIG. 2 shows an example device 20 that has a construction for achieving a stronger reflection in the optical cavity.
Referring to FIG. 2, the device 20 has a construction similar to the device 10 in that the device is constructed on a substrate 21 corresponding to the substrate 11. On the substrate 21, there is provided a buffer layer 22 of n.sup.+ -type InP, and an active layer 23 corresponding to the active layer 12 is provided on the buffer layer 22. Further, a contact layer 24 corresponding to the contact layer 13 is provided on the active layer 23 and a ring shaped electrode 26 is provided on the contact layer 13 in correspondence to the electrode 15. The active layer 23 as well as the contact layer 24 are surrounded laterally by an insulator layer 25 that corresponds to the insulator layer 14.
In the device 20 of FIG. 2, there is provided an opening 21a on the lower major surface of the substrate 21 such that the opening 21a exposes the lower major surface of the buffer layer 22. Thereby, there are formed two opposing mirror surfaces, one in correspondence to the exposed upper major surface of the contact layer 24 and the other in correspondence to the exposed lower major surface of the buffer layer 22. By setting the thickness of the layers 22, 23 and 24 appropriately, it is possible to obtain a strong laser oscillation by the device of FIG. 2.
FIG. 3 shows another conventionally proposed laser diode 30 that produces the optical beam in the direction perpendicular to the epitaxial layers.
Referring to FIG. 3, the laser diode 30 is constructed on a substrate 31 corresponding to the substrate 21 of the device 20 of FIG. 2, and there is provided a buffer layer 32 corresponding to the buffer layer 22 on the substrate 31. On the buffer layer 32, there is provided an active layer 33 similar to the active layer 23 except that the active layer 33 covers the entire upper major surface of the buffer layer 32. Further, a contact layer 34 corresponding to the contact layer 24 is provided to cover the entire upper major surface of the active layer 34. On the upper major surface of the contact layer 34, there is provided an ohmic electrode 36 while there is provided an ohmic electrode 37 on the lower major surface of the substrate 31.
Thus, the device 30 of FIG. 3 has a structure similar to the usual laser diode that uses the cleaved surfaces for the optical cavity, except that there is provided an oblique groove 35 that cuts the active layer 33 with an angle of 45 degrees. There, the optical beam that is produced in the active layer 33 is bent perpendicularly upon reflection at the surface of the groove 35 and the optical beam exits in the direction perpendicular to the upper major surface of the layer 34. Thereby, the cleaved surface b and the upper major surface a of the layer 34 act as the mirror surfaces for reflecting the optical beam back and forth for the optical amplification. Obviously, this device requires the cleaving process in order that the device becomes operational. In other words, device 30 is not suitable for the testing in the as-formed state.
In any of the devices 10 and 20, it should be noted that the active layer 12 or 23 has a reduced diameter in the order of 4-10 .mu.m for concentrating the carriers therein. Thereby, although such a construction may be preferable for decreasing the threshold of laser oscillation, there arises a problem that the optical beam tends to be spread with a large solid angle. In other words, it is difficult with these devices to obtain a highly coherent parallel optical beam that is preferable for various applications. Further, such a spreading of the optical beam occurs also in the buffer layer 22 when the optical beam exits from the active layer that is surrounded by the insulator layer 25 that has a low refractive index. It should be noted that the optical confinement achieved in the active layer by the insulator layer 25 does not occur in the buffer layer 22 that lacks the low refractive index layer. Thereby, the divergent optical beam is spread each time it is reflected back at the exposed lower major surface of the buffer layer 22, and the efficiency of laser oscillation is inevitably deteriorated.